The present invention relates to viscous couplings, and more particularly, to such couplings of the type which are now employed in automatic transmission torque converters, to serve as by-pass elements, by-passing the torque converter.
Torque converter type automatic transmissions have achieved widespread application and acceptance in motor vehicles. Although generally satisfactory in this application, torque converter type automatic transmissions inherently involve a certain amount of slippage, and therefore, inherently result in losses in vehicle fuel economy. In an effort to minimize such slippage, and thereby increase fuel economy, efforts have been made to by-pass the torque converter with some type of relatively direct drive, which typically is activated when the vehicle is operating in the higher gear ratios, and above a predetermine vehicle speed. Although the term "by-pass" is used in regard to this type of element, those skilled in the art will understand that the element is actually transmitting torque in parallel with the torque converter.
While such prior art direct drive by-pass arrangements have resulted in improvements in fuel economy, certain types of by-pass elements utilized have, under certain operating conditions, transmitted various driveline vibrations to the passenger compartment of the vehicles, causing degradation of the ride quality of the vehicle.
In an effort to provide a by-pass element which would not transmit driveline vibration to the passenger compartment, those skilled in the art have utilized a viscous coupling as the by-pass element. Although the use of a viscous coupling as the by-pass element does serve to minimize the transmission of driveline vibration, it is still important that the coupling be designed for maximum durability and, at the same time, be reliable and inexpensive to manufacture.
A typical prior art viscous coupling of the type to which the present invention relates is illustrated and described in U.S. Pat. No. 5,044,477, assigned to the assignee of the present invention, and incorporated herein by reference. Such couplings typically include a cast aluminum body or sidewall member, and a cast aluminum clutch member. Each of these cast aluminum members is then machined to define a plurality of annular, concentric lands and grooves, arranged so that the lands and grooves of the body and clutch are interdigitated, upon assembly thereof, to define a serpentine-shaped viscous shear space. During operation, the shear space is filled with viscous fluid, which is typically a silicone fluid, such that rotation of the body member transmits torque to the clutch member by means of viscous shear drag, as is now well known to those skilled in the viscous coupling art.
In designing and applying such viscous couplings, for use as torque converter by-pass elements in a typical vehicle application, there are usually two competing design considerations. On the one hand, it is desirable to minimize the amount of slip (i.e., the difference between the input speed and the output speed) so that, during operation at normal driving speeds, the overall fuel efficiency of the vehicle is as high as possible. On the other hand, the amount of slip and the dampening by the viscous coupling must be sufficient to minimize the amount of vibration transmitted by the clutch, and provide the desired ride characteristics.
One of the primary design characteristics of such a viscous coupling is the "drive factor", which is typically defined as the effective surface area under shear. The drive factor of a viscous coupling is a function of the surface area that is being sheared, the clearance between adjacent sheared surfaces, the radius at which the sheared surfaces act, and the fluid viscosity of the active fluid volume within the shear chamber.